In the electrowinning and the electrorefining of metals and in electroplating it is known to add so-called inhibitors to the electrolyte in order to ensure a more uniform deposition of the metal so that the quality of the metal-deposits will be greatly improved as non-uniform and rough surfaces, which promote the formation of inclusions, will be avoided.
The concentration of the inhibitors is usually less than 10 mg/l.
Inhibiting additives in electrolytes used to electrorefine copper may consist, e.g. of thiourea and of substances of high molecular weight, such as animal glue, lignin sulfonate or aloe extract.
Particularly in electroplating, synthetic inhibitors or brighteners have also been used. In electrolytes used to electrorefine copper, thiorea and animal glue are often used as inhibitors in concentrations below 10 mg/l.
A significant technical problem with such systems is that the effective concentration of the additives decreases in the course of the process because the additives are consumed or decomposed or are included in the metal deposits or because the substances, which often have a high molecular weight, are degraded to substances which have a low molecular weight and a low activity.
For this reason it is of considerable importance to be able to detect the effective additives which are still present and to supply makeup additives to the electrolyte in the required quantity before the quality of the deposition of metal is adversely affected.
A knowledge of the rate at which an additive for improving the deposition of metal is decomposed is also highly significant for the design of an industrial plant because in units for a continuous processing the inhibitor which has initially been added may have been decomposed before reaching the outlet of the unit so that the size of the unit for continuous processing must be selected in dependence on the rate of decomposition.
For this reason efforts have been made in the prior art to objectively determine the concentration of the additives in the winning and refining of metals and in electroplating so that the usual merely visual inspection of metal coatings will no longer be required. For instance, U.S. Pat. No. 4,474,649 discloses a polarographic method of determining thiourea in copper-containing electrolytes at concentrations up to less than 1 mg/l.
Whereas that method is used for a direct analytic determination of the concentration of the additives, most efforts are directed to the determination of the concentration of substances of high molecular weight to measure the effect of said substances. In that connection, potential measurements are of high significance.
It has been found that the cathode potential can distinctly be increased by the macromolecular additions. The publication by I. H. Warren (ed.) "Application of Polarization Measurements in the Control of Metal Deposition", Elsevier (Amsterdam) 1984, discloses on page 84 a measuring cell for use in the electrowinning of zinc. In that cell the potential at a fresh aluminum surface represented by a moving aluminum wire is continuously measured against a calomel electrode (reference electrode) at a very low current density (0.5 mA/cm.sup.2). The electrolyte solution must be filtered before measurement in order to prevent a clogging of the diaphragm.
But the measurement of only the stationary cathode potential has the disadvantage that it will not definitely measure the concentration of the inhibitor in a complex electrolyte composition as is used in industrial practice.
The measurements will be influenced by various factors, such as the acid concentration, the metal ion concentration and the presence of impurities. For this reason the measurement of the stationary cathode potential can be performed only after a careful calibration and if the composition of the electrolyte is substantially constant. Besides, a reference electrode, such as a calomel electrode, is required in the known methods and its diaphragm is liable to be clogged in an electrolyte of the type used in industrial practice, particularly if precipitation must be expected during an electrolysis.